J-W Nah et al., ‘A Study on Coining Processes of Solder Bumps on Organic Substrates,’ IEEE Transactions on electronics packaging manufacturing, Vol. 26, No. 2, April 2003, p. 166. shows a current high volume manufacturing method of forming solder bumps on laminates. The solder paste stencil printing method has been used for forming solder bumps on organic substrates for a long time. An organic substrate is provided with a plurality of pads and a solder resist layer. A stencil mask is aligned therewith. Solder paste is dispensed through the holes in the mask using a squeeze knife or blade. The mask is removed, leaving upstanding solder paste regions. Solder reflow causes the formation of rounded bumps with flux residue thereon. The residue is cleaned away leaving solder balls with a projecting height above the resist. In summary, solder paste is stencil printed through a mask aligned to the substrate solder resist (SR) openings. The mask enables deposition of solder paste material that stands above the SR after mask removal. However, commercial applications of the solder paste stencil printing method are limited to substrates with pitches greater than or equal to 150 microns because the high volume percentage of flux (around 50 volume %) in the solder paste hinders formation of high volume solder bumps without solder bridging.
U.S. Pat. No. 6,213,386 of Inoue et al., entitled ‘Method of forming bumps,’ discloses wherein solder balls and a tool having a large number of through-holes are used, and under the condition that the through-holes of the tool are aligned with the pads of the semiconductor device, the solder balls are charged into the through-holes, pressed to be fixed on the pads which have an adhesive such as flux, and then reflowed to form bumps.
A pre-formed solder ball mounting method for fine pitch applications under 150 microns pitch has been employed. An organic substrate is provided with a plurality of pads and a solder resist layer. A mask for flux is aligned therewith. Tacky flux is applied and the mask with adherent flux is removed, leaving flux portions on the pads. A mask for solder ball dispensing is aligned with the pads. Preformed solder balls are located in openings in the mask, in contact with the flux portions which retain them when the mask is removed. Reflow and flux cleansing are then performed. This so-called micro ball mounting method accordingly employs two masks, one to dispense tacky flux and the other to place preformed solder balls on the pads of the substrate. The tacky flux makes the balls stick to the bottoms of the SR openings during separation of the mask and before the reflow.
U.S. Pat. Nos. 7,291,517 and 7,569,164 disclose solder bumping methods wherein dry film resists are applied over solder resist layers, leaving electrodes on the substrate exposed. A solder paste composition fills the spaces defined by the solder resist and film over the electrodes. Solder balls are formed upon heating to an appropriate temperature. Because solder paste is comprised of solder powder and flux, the heights of the solder balls formed in such processes are likely to be less than the thickness of the film even though the powder and flux may initially be equal to the thickness of the film.
U.S. Pat. No. 7,410,824 describes a method in which a passivation layer on a chip has bond pad openings that expose UBM layers formed on bond pads. A film is formed and patterned to have openings centered over the bond pad openings. A metal stencil having openings is formed over the film. The stencil openings are larger than the openings in the film which are, in turn, larger than openings in the passivation layer. Solder paste is screen printed onto the UBM layers through the openings and reflowed to form solder balls. After the metal stencil is removed, the film is stripped from the substrate followed by a second reflow.